بررسي آزمايشگاهي تثبيت كننده پليمري پلي وينيل استات، بر فرسايش داخلي و مقاومت ماسه رس دار كائولينيتي

Investigation of Effectiveness of polyvinyl acetate polymer stabilizers on Internal Erosion and strength of kaolinity clay sand

فرسايش داخلي يكي از متداول ترين عوامل شكست سدهاي خاكي مي باشد و با توجه به حجم عظيم آب ذخيره شده در پشت سدها، هرگونه تخريب يا گسيختگي در سدها منجر به عوارض مخربي چه از نظر مالي و چه از نظر جاني مي شود. در راستاي كنترل فرسايش داخلي و اصلاح خاك هاي مستعد فرسايش، نكته حائز اهميت استفاده از تثبيت كننده هاي مدرن و ايمن به جاي تثبيت كننده هاي سنتي و مضر مي باشد. در اين پژوهش از ماده پليمري پلي وينيل استات جهت اصلاح فرسايش پذيري ماسه رس دار كائولينيتي استفاده شده است. بدين منظور ماسه رس دار كائولينيتي به صورت مجزا و سپس تركيب با درصدهاي مختلف پلي وينيل استات، در هدهاي مختلف مورد آزمايش فرسايش قرار گرفت. نتايج نشان داد كه فرسايش پذيري ماسه رس دار كائولينيتي بسيار سريع مي باشد كه با افزودن پلي وينيل استات به آن، فرسايش به شكل قابل مشاهده اي كاهش يافته و از نظر كيفي فرسايش پذيري در گروه تقريبا و بسيار آهسته قرار مي گيرد. ضمن آنكه افزودن پلي وينيل استات به ماسه رس دار كائولينيتي باعث افزايش 60 درصدي مقاومت فشاري آن گرديد.

The two most common causes of embankment failure are embankment overtopping and internal erosion. Internal erosion of soil resulting from seepage flow is the main cause of serious hydraulic work (dykes, dams) failure, in terms of the risk of flooding areas located downstream. The erosion characteristics are described by the Erosion Rate Index, which measures the increase of erosion rate with respect to an increase in the hydraulic shear stress; and the Initial Shear Stress, which represents the minimum hydraulic shear stress when erosion starts. Values of the Erosion Rate Index span from 0 to 6, indicating that the changes in erosion rates in response to changes in hydraulic shear stress can differ by up to 106 times across different soils. Coarsegrained, noncohesive soils, in general, erode more rapidly and have lower Initial Shear Stresses than finegrained soils. The challenge in predicting failure due to internal erosion is characterizing the material properties relevant to the rate of failure. Therefore, it is very important to improve the erosion resistance of soils using appropriate and cost effective techniques. In order to control internal erosion and treated erodbile soil, important point is use of modern stabilizers instead of traditional stabilizers which is harmful. In this study, polyvinyl acetate polymer material has been used for treatment erodibility of kaolinity clay sand. To conduct this research, kaolinity clay sand has been treated with different percentages of polyvinyl acetate polymer and it has been tested with hole erosion apparatus in different hydraulic gradients. The hole erosion test (HET) is one of several available procedures for characterizing the erodibility of cohesive soils that might be susceptible to internal erosion investigations of dams and levees. It was first developed in a constantflow configuration (Lefebvre et al. 1984) and more recently in a constanthead configuration by Wan and Fell (2004). The HET utilizes an internal flow through a hole predrilled in the specimen, a flow condition similar to that occurring during piping erosion of embankment dams. In the constanthead configuration, the test head is typically doubled, starting from 50 mm, until progressive erosion of the predrilled hole is produced. Measurements of accelerating flow rate through an eroding predrilled hole in a test specimen yield estimates of the critical shear stress and erosion rate coefficient. The initial and final eroded hole diameters are used to compute initial and final friction factors, and intermediate hole diameters are then computed from flow rates measured during the course of the test. The result showed that erosion rate of kaolinity clay sand is extremely rapid and polyvinyl acetate polymer stabilizer increased the resistance of kaolinity clay sand to erosion. It was also found that with the addition of polyvinyl acetate polymer and its description has been changed from extremely rapid to moderately slow erosion. While adding polyvinyl acetate polymer kaolinity clay sand caused the increase 60 percent unconfined compression strength.